Silicon-based materials, where silicon is the primary material of construction, are employed in numerous integrated circuits (IC) and microelectromechanical systems (MEMS) devices. However, it has long been known that in aqueous chemical environments, where silicon-based sensors and actuators may be used, that corrosion (etching) of the silicon-based materials can cause premature device wear and failure. In fact, there are many commonly used processes for machining silicon that rely on wet corrosion (etching) of silicon; see for example Kendall, D. L.; Shoultz, R. A. “Wet Chemical Etching of Silicon and SiO2, and Ten Challenges for Micromachiners”, SPIE Handbook of Microfabrication, Micromachining, and Microlithography, Vol. 2, SPIE Optical Press, pp. 41-97, 1997. Ed. P. Rai-Choudhury. Recently, MEMS technology has been applied to fluid management systems. An example of a microfluidic fluid management system that incorporates silicon-based MEMS devices is continuous ink jet (CIJ) printing.
Continuous ink jet (CIJ) printers typically consist of two main components; a fluid system and a printhead, or multiple printheads. Ink is pumped through a supply line from a supply reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head. Stimulations are applied to the printhead to cause those ink streams to form streams of uniformly sized and spaced droplets, which are deflected into printing or non-printing paths. The non-printing droplets are returned to the supply reservoir via a droplet catcher and a return line. U.S. Pat. Nos. 3,761,953 A, 4,734,711 and 5,394,177 and EP 1,013,450 describe in detail the design of a fluid system for CIJ apparatus. The more recent development of a silicon-based MEMS CIJ printhead fabrication and printing apparatus can be found in U.S. Pat. No. 6,588,888 and U.S. Pat. No. 6,943,037, the disclosures of which are herein incorporated by reference. The design of the nozzle plate (printhead die) used in the drop generator of the printing system is one of the distinguishing elements of MEMS CIJ technology. A single crystal silicon die may be used as the substrate for the nozzle plate and, complementary metal oxide semiconductor (CMOS) electronics are included as part of the device. The surface nozzle structures and associated on-board CMOS electronics are fabricated using the same manufacturing technologies and material sets employed for the construction of silicon integrated circuits. The printhead die also incorporates fluid channels running through the silicon. During drop generation, heaters in the device transfer thermal energy to the fluid jetting through each nozzle.
As noted in the discussion above, the CIJ printhead is comprised of several components. A more detailed discussion of the printhead and its operation is provided herein with particular emphasis on silicon and its interactions with fluids, given that silicon-fluid interactions are particularly relevant to the present invention. These components include a manifold for interfacing with the fluid system and accepting ink or other fluids supplied by the fluid system to allow transport of these fluids to other components of the printhead; an electrical interconnect system means for interfacing with the electrical signals supplied by an external writing system that supplies the printhead with the information pertaining to the drop-wise formation of a printed image on a support, where the support is stationary or non-stationary, from ink-containing drops generated by the printhead; and a drop-generating component, whose function is to provide a means for generating drops from ink or other fluids delivered to the drop generating component from the manifold. The drop-generating component providing a means for generating drops in a silicon-based CIJ printing system employs silicon-based devices fabricated using the same technology employed for fabricating silicon integrated circuits. The silicon-based devices may contain multiple fluid channels as well as a plurality of small orifices, also called nozzles, which enable ink or other fluids supplied by the fluid system to pass from the manifold to the support through the formation of one or more columns of fluid also called fluid jets, which exit the silicon-based device when appropriate pressures are employed. The fluid column(s) or fluid jet(s) transform into well-defined drops under appropriate conditions. The pressures employed in silicon-based CIJ printing system are generally above 69 kPa and less than 1380 kPa. The materials of construction of the silicon-based devices in a silicon-based MEMS CIJ printhead may be quite varied and the materials of construction that contact ink or other fluids supplied by the fluid system or manifold are of particular interest to the present invention.
Silicon-based devices used as components that provide a means for generating drops from a fluid are generally fabricated using substrates prepared from single crystal silicon. The use of large grain polycrystalline silicon substrates for device fabrication is known in the art. The substrates may have varying thicknesses, from 50 microns to greater than 1 mm, and the substrate surface may have any crystallographic orientation that is suitable for the device application. For example, the silicon substrate may be prepared with an orientation defined by Miller indices of <100>, <111>, <110>. The use of various crystallographic orientations in device substrates is well known to those familiar with the art of semiconductor device fabrication. The singe crystal silicon substrate may have varying electrical properties. For example, the electrical properties of the single crystal silicon can be varied by the incorporation of small amounts of foreign impurities, also called dopants or carriers. These foreign impurities, such as, for example, boron or phosphorus, determine whether the electrical charge of the majority carrier type in the silicon crystal is negative or positive. Such modified substrates are known as n-type and p-type silicon, respectively The use of both p and n-type silicon substrates for fabrication of silicon-based devices is known in the art. The use of silicon substrates of low resistivity, where the resistivity is less than 100 ohm-cm, and the use of silicon substrate of high resistivity where the resistivity is greater than 1000 ohm-cm, irrespective of carrier type and substrate crystallographic orientation, is known in the art of semiconductor device fabrication.
The additional preparation of substrates by deposition of layers of silicon, either polycrystalline or amorphous by various means as well as deposition of silicon by various means on insulating layers prepared by various means, such as, for example, polysilicon deposited on silicon dioxide insulators formed by thermal oxidation of the silicon substrate, also known as silicon on insulator or SOI, is known in the art. The resulting deposited silicon containing layer(s) may be either doped or undoped, p-type or n-type, and additionally may be either polycrystalline, meaning that the arrangement of silicon atoms in three dimensional space within the layer are identical with those found in single crystal silicon, or amorphous or poorly crystalline, meaning that that the arrangement of silicon atoms in three dimensional space within the layer deviates relative to those found in single crystal silicon and shows varying degrees of disorder relative to those atomic positions found in single crystal silicon. Device performance has been shown to improve after substrate surface quality has been controlled by the use of additional layer deposition, and this observation is familiar to those knowledgeable in the art of semiconductor device fabrication.
The use of subsequently deposited layers optionally containing silicon is known in the art of semiconductor device fabrication. Deposited layers optionally containing silicon can be prepared by any method known in the art of semiconductor device fabrication including chemical vapor deposition with the optional use of plasma assistance or enhancement at low (<400° C.) and high temperatures (>400° C.) under both low pressure (<1 torr) and high pressure (>1 torr) conditions. Deposited layers optionally containing silicon can be prepared by vapor deposition by physical vapor deposition (evaporation) optionally plasma assisted or enhanced, as well as by epitaxial growth methods. The resulting optionally silicon containing layers may be electrically insulating or electrically conductive to varying degrees, either doped or undoped, p-type or n-type, and additionally may be either polycrystalline, meaning that the arrangement of atoms in three dimensional space within the layer are identical with those found in single crystals of the same elemental composition, or amorphous or poorly crystalline, meaning that that the arrangement of atoms in three dimensional space within the layer deviates relative to those found in single crystal of the same composition and shows varying degrees of disorder relative to those atomic positions found in single crystal silicon. It is known in the art that silicon containing deposited layers may contain additional foreign atoms of varying amounts including, for example, some of the aforementioned dopants boron and phosphorus to control electrical properties, and additional atoms, interstitial or otherwise, resulting from the deposition process or a combination thereof Examples of dopants include boron, phosphorus, arsenic, nitrogen, carbon, germanium, aluminum, and gallium. Examples of interstitial or non-interstitial foreign atoms include hydrogen, oxygen, nitrogen, carbon, select atoms from elements listed from group VI B of the periodic table (O, S, Se, Te) and select atoms of elements listed in the group VII B of the periodic table (F, Cl, Br, I). Hydrogen, oxygen, nitrogen, and carbon are commonly present with silicon in devices and devices containing microelectromechanical systems and each of the elements oxygen, nitrogen and carbon are often found combined with silicon in the form of stoichiometric or non-stoichiometric binary, ternary, and quaternary compounds like silicon hydrides of varying compositions, silicon oxides of varying compositions and hydration including silicon suboxides and hydrated silicon oxides and suboxides, silicon nitrides of varying compositions, silicon oxynitrides of varying compositions, silicon carbides of varying compositions, and silicon oxycarbides of varying compositions. These binary and ternary silicon containing compounds can be either discrete layers in the device or part of the surface composition of silicon, polysilicon, and amorphous silicon. Additionally, other elements such as Al, Ti, Ta, W, Zr, Hf, and Cu are often found with silicon and/or silicon containing binary compounds such as silicon oxides and silicon carbides, in devices and are sometimes observed as intermetallic alloys with silicon. Examples of intermetallic silicon containing alloys are titanium containing silicides of all compositions, tantalum containing silicides of all compositions, tungsten containing silicides of all compositions, zirconium containing silicides of all compositions, halfnium containing silicides of all compositions, copper containing silicides of all compositions, as well as ternary aluminum silicon oxides, ternary halfnium silicon oxides, ternary zirconium silicon oxides. Those knowledgeable in the art of semiconductor device fabrication are familiar with the different alloys, binary compounds, ternary and quaternary compounds that can form during processing and this is considered common knowledge in the art.
When a continuous inkjet printing system is in operation, fluid is essentially always flowing through the nozzles of the drop generator. There may be startup fluids passing through the printer for cleaning the fluid delivery system before printing with inks. Inks may remain in the printing system for extended times during a given printing run because the run duration may vary from hours to weeks. Flushing fluids may be used during ink changeovers or as part of routine maintenance. When the system is printing, only a small portion of the ink passing through the drop generator actually prints on the substrate. Most of the ink is collected and returned to the fluid delivery system for reuse. Finally, shut down fluids and storage fluids may be used to clean out inks from the fluid delivery system and the printhead, and ensure that the system does not fail during startup after storage.
It is desirable to have a printhead operate reliably for many hundreds to thousands of hours. The fluid volume passing through a CIJ print head is large; accordingly, over a desired printhead lifetime, many thousands of liters of solution can pass through the printhead die. Therefore there is extensive exposure of the silicon-based nozzle plate to fluids in CIJ systems. Any degradation of the silicon-based materials in these solutions, as by corrosion (or etching, or dissolution), represents a great concern.
There is a substantial pressure gradient across the continuous ink jet printhead nozzle plate during operation that can be 100's of kPa, putting the fragile device under great stress. Corrosion of the silicon-based substrate can lead to complete rupture of the printhead die itself, or minimally to increases in the size of the channels and orifices through which the ink flows, creating drop ejection defects such as permanently crooked jets or erroneous drop sizes. Extensive corrosion of the backside of the device can alter the thermal mass of the backside die and compromise heat management within the device leading to additional potential problems around drop formation from the jets. Clearly, corrosion of silicon-based materials needs to be prevented or minimized in silicon-based MEMS CIJ printing and in other applications where silicon is exposed to solutions that may corrode the silicon. An approach to addressing the problem of silicon-based device corrosion is to apply passivation coatings to the device. Passivation coatings are protective coatings that typically exhibit relatively low rates of etching. Silicon itself is known to readily form native silicon oxide coatings; however, these thin native silicon oxide coatings (ca. 1 nm) are also subject to corrosion processes and can be insufficient to protect the silicon metal. Examples of passivation coatings include thermally produced silicon oxides, various silicon nitrides, and tantalum oxide (G. F. Eriksen and K. Dyrbye, “Protective Coatings in Harsh Environments,” J. Micromech. Microeng. (1996), vol. 6, 55-57; C. Christensen et al., “Tantalum Oxide Thin Films as Protective Coatings for Sensors,” J. Micromech. Microeng. (1999), vol. 9, 113-118.) However, the passivation approach is problematic because it requires the introduction of additional coating steps to the process, the coatings can introduce undesirable effects in the device such as stress, and coating defects like pinholes can compromise the effectiveness of the passivation coating. Moreover, many coating methods may not be practical for microfluidic devices, because the areas to be coated, such as fluid channels, are internal to the device or because the coating methods require conditions, such as a temperatures, which are not compatible with the device. Another general approach to improving ink performance with regard to silicon corrosion is through adjustment of the ink pH value through the use of appropriate buffer solutions. For example, Inoue et al. in U.S. Pat. No. 7,370,952 B2 note that buffers can be used to adjust the pH values of inks used in drop-on-demand inkjet printers to reduce the effects of corrosion. This is primarily because the corrosion of silicon is known to be accelerated by higher pH value (more alkaline) solutions, such as those used in wet etching processes. At the same time, compositions useful to inkjet inks often require some alkalinity in order to maintain solution integrity, e.g., in order to prevent precipitation of ink components. However, for technologies such as CIJ, even reduced etch rates, i.e., in the sub 100 nm/h range, can prematurely degrade system performance after just tens to hundreds of hours of operation, even resulting in catastrophic device failure. The direct measurement of silicon corrosion rates, also referred to as etch rates, has been disclosed by Dockery et al. in U.S. Pat. App. Pub. No. 2009/0065478 A1, the disclosure of which is incorporated herein by reference.
The use of organic naphthalenic azo compounds for use as stabilizers in the preparation of colloidal gas black suspensions, also known as pigment dispersions, for subsequent use in the production of inks, inkjet inks, surface coatings, and colored printing inks, is taught by Zoch et al. in U.S. Pat. No. 7,160,377 B2. Such reference does not teach their use as silicon corrosion inhibitors added to pigments predispersed without such azo compounds, and it does not teach their use with other pigments or in essentially pigment-free solutions. Furthermore, such reference is limited to naphthalenic azo compounds, and it is inconvenient because the azo compounds are added to the pigment dispersion prior to ink formulation.